The invention relates to a microwave oven comprising an oven cavity bounded by a plurality of conductive walls, a microwave source mounted external of said oven cavity for feeding microwave energy into the interior of the oven cavity and a bottom plate of insulating material situated above the conductive bottom wall of the oven cavity and adapted to support food to be heated. The feed for the microwave energy is situated above the bottom plate, preferably in the top wall of the oven cavity. The most common manner to excite such an oven is to produce and maintain several simultaneously existing resonant modes in the oven cavity according to the so called multi-resonance principle.
An essential problem in multi-resonant oven cavities with feeding from above is that, in case of extended objects (loads) to be heated which have such a large thickness that microwave fields coming from above are not able to penetrate sufficiently deep, there will not be heating of the central parts of the lower portion of the load. In ovens with microwave feeding from below this problem is solved, but in this case the microwave field passing upwardly from the region beyond the load and then being reflected against the cavity walls will generally be in sufficient to heat the central upper parts of the peripheral parts of the load because of diffraction phenomena determined by the load geometry, the so-called edge heating effect.
Many attempts at solving the above-mentioned problem have been made. A radical solution is to feed the microwaves both from above and from below with the aid of separate waveguide systems. Such a solution will, however, be expensive. Other solutions involve various ways of "receiving" and converting microwave energy by means of special structures or systems in order to improve the heating of the lower part of the load.
Still other methods for improving the heating effect in multi-resonant oven cavities are based upon the idea to locally change the oscillation pattern in the cavity in a controlled manner. Reference may be made to the following prior art documents:
U.S. Pat. No. 3,740,514 describes a method for modifying resonant modes in an oven cavity or for providing additional resonant modes by means of dielectric blocks positioned so as to achieve a more even distribution of the microwave energy throughout the load. However, the dielectric bottom plate supporting the load is not used for this purpose.
U.S. Pat. No. 4,121,078 describes a microwave oven in which at least a part of the output of the microwave source is coupled to a surface waveguide system, substantially of a delay line type, that acts to concentrate microwave energy in a specific region of the oven. The surface waveguide system comprises a periodic structure of a metallic material and the food-supporting bottom plate of low-loss dielectric material does not perform a specific microwave function, i.e. it does not modify the microwave field in dependence on the wavelength.
U.S. Pat. No. 4,019,009 describes several variants of the above techniques. All these variants relate to arrangements comprising both dielectric and metallic elements for providing a structure that is referred to as a surface waveguide, but should be more truly regarded as a leaky delay line structure.
U.S. Pat. No. 3,941,968 describes a microwave browning and searing plate comprising a periodic array of parallel dielectric bar members, each having a coating of conductive material on three sides and an uncoated top side that supports the food. When microwave energy is applied to the plate, the uncoated sides provide an intense fringing field adjacent to the top surface of the plate for browning and searing the outer surfaces of the food. The microwave field existing in a multi-resonant microwave oven cavity provides the microwave energy that is applied to the browning plate. A structurally and functionally similar browning plate is described in U.S. Pat. No. 3,857,009 in which the periodic array comprises parallel strips of alternately higher and lower dielectric constant.
U.S. Pat. No. 4,165,454 describes a more generalized microwave energy feeding system that may act as both a microwave delay line and a resonance structure determining the microwave field in its vicinity. This structure is essentially a wire or strip conductor configuration adjacent to the bottom wall of the microwave oven and the excitation of the oven cavity takes place by directly supplying microwave energy to the structure.